Site Editor: remove the need for postId and postTemplate args

[mattwiebe]

Now that WordPress/gutenberg#49861 has landed on wpcom, we can remove our workaround of adding the query args ourselves.

Related to #75183 and WordPress/gutenberg#49861

Proposed Changes

• Remove the call to /sites/SITE/block-editor in the /site-editor/SITE redirect.
• The wpcom request was also generating some improperly-handled errors, so good to get rid of it.

Testing Instructions

Try accessing /site-editor/SITE with this branch built. You should see no query args in the redirected Site Editor URL (other than calypso_origin if running locally).

Pre-merge Checklist

• Has the general commit checklist been followed? (PCYsg-hS-p2)
• Have you written new tests for your changes?
• Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• Have you checked for TypeScript, React or other console errors?
• Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• Have we added the "[Status] String Freeze" label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?
• For changes affecting Jetpack: Have we added the "[Status] Needs Privacy Updates" label if this pull request changes what data or activity we track or use (p4TIVU-ajp-p2)?

[github-actions]

Calypso Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-64297

Jetpack Cloud live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-64297&env=jetpack

[matticbot]

Here is how your PR affects size of JS and CSS bundles shipped to the user's browser:

Sections (~40 bytes removed 📉 [gzipped])

name              parsed_size           gzip_size
gutenberg-editor       -134 B  (-0.0%)      -40 B  (-0.0%)

Sections contain code specific for a given set of routes. Is downloaded and parsed only when a particular route is navigated to.

Legend

What is parsed and gzip size?

Parsed Size: Uncompressed size of the JS and CSS files. This much code needs to be parsed and stored in memory.
Gzip Size: Compressed size of the JS and CSS files. This much data needs to be downloaded over network.

Generated by performance advisor bot at iscalypsofastyet.com.